Federer teamed with Belinda Bencic to win the match for Switzerland in straight sets over the United States' Williams and Francis Tiafoe, 4-2, 4-3 (5/3), to clinch the deciding rubber under the Fast4 format.
Federer and Williams, holders of a combined 43 singles Grand Slam crowns, won their singles matches in the tournament, which set up the doubles clash, and both players left the court seemingly very pleased with the experience.
"I mean the guy is the greatest of all time, both on and off the court", she said.
"Both on the court and off the court has such charisma and he's such a wonderful player".
Federer and Williams shared a good-natured interview afterward and engaged in a selfie. "I'm sad it's over, I was just warming up", Williams said of her match against the Swiss master.
But the 23-time Grand Slam champion levelled up to force a decider, winning the second set 6-4, and eventually triumphing 4-6, 6-4, 6-3 - meaning the mixed doubles match would decide the tie.
"It was definitely a relief", Williams said of the result.
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Federer also posted the picture on social media, with the caption, "Oh what a night". It was super cool.
'For me it was a bit nerveracking too be honest all of a sudden I was thinking you have to win this point but then you're telling yourself it's Serena Williams. "The Goat won." She used the goat emoji as a stand-in for the acronym GOAT, meaning "Greatest of All Time". "You just can't read it".
"I was telling myself, 'This is Serena Williams, this is what I've always wanted, a big-time moment like this'. I hope tennis fans tune in and watch it because it's going to be one time and probably never again in this type of competition", Federer told the press, adding that the two "can relate to each other more so now, being parents". "I was so excited, and literally it was the match of my career".
The New Year couldn't be off to a more legendary start for the world of sport, and the world can be forgiven for nearly not noticing there were two players on the court.
"I think his serve is super-underestimated".
Defending champion Switzerland will qualify for Saturday's final if it beats Greece on Thursday.